Bullet Handling kit, because personally I think the ride quality of my V6 mustang is horrible and that's not even taking into effect the constant squeeks....
Just doing swaybars (stiffer in the front, and adding one to the rear) works miracles for the V6's handling. The shocks, springs, and struts would be an improvement too, but not as dramatic. The stock V6 springs and shocks are already decent for street driving. The biggest problem they have is how they give the car that 4x4 look.